This invention relates to a pouring pad adapted to be mounted in a tundish so as to receive the impact of a stream of molten metal to be poured into the tundish. The pouring pad of this invention has at least one upwardly projecting ridge of a configuration designed to suppress turbulence within the metal bath in the tundish.
In the continuous casting process, molten steel is supplied to the casting machine in heat lot batches produced at the steelmaking facilities of the plant. Each batch is contained in a ladle which receives molten steel from a furnace and is then transferred to the casting machine. The ladle is positioned over a tundish at the casting machine. Molten steel is then poured through a nozzle in the bottom of the ladle into the tundish. When one ladle is emptied, it is moved away and another full ladle is transferred into position over the tundish. During ladle transfer, molten steel continues to be withdrawn from the tundish into the casting machine. Consequently, the level of molten steel in the tundish drops until another full ladle is positioned and pouring into the tundish has begun again. The purpose of the tundish is to absorb these discontinuities in the supply of molten steel during ladle transfer and to supply a constant stream of steel to the continuous casting machine.
During the period when pouring of molten steel has begun again after ladle transfer, there is an abrupt increase in the pouring rate from the new ladle to the tundish at a time when the level of molten steel in the tundish is lower than normal (e.g., 10% to 25% less than normal). The impact of the pouring stream with the lower level of molten steel in the tundish causes increased turbulence at the slag-metal interface in the tundish. This leads to entrainment of tundish slag with the molten steel as it is withdrawn from the tundish into the casting machine. In addition, the high interfacial turbulence makes it impossible to maintain a protective cover on the steel in the tundish and to protect it from severe reoxidation when casting aluminum killed steel grades. The result is a degradation in steel cleanliness which can require the downgrading of slabs cast during the ladle change period from critical applications. Cleanliness requirements for drawn and ironed and exposed automotive applications, for example, are very high. Hence from a total of 6 to 7 slabs cast from each heat, the downgrading of 2 slabs cast during the ladle change period decreases the availability of as-cast material for these critical applications (not including subsequent diversions) to about 70 percent. In order to overcome these problems, some steel producers have replaced the ladle-tundish-mold system with a ladle-ladle-mold or ladle-ladle-tundish-mold or ladle-tundish-tundish-mold system. These systems are, however, difficult and expensive to implement since they require major plant layout modifications and the addition of extra facilities or new equipment capabilities. Consequently, there is a great need for alternate means for improving steel cleanliness during non-steady pouring conditions within the framework of the ladle-tundish-mold casting system.
German Offenlegungsschrift 2,643,009 discloses a refractory anti-splash grid for a tundish used in the continuous casting of steel. The grid has a honeycomb of box-shaped channels open at both the top and bottom. The thickness of the grid is from 10 to 200 mm (preferably 40 to 100 mm). The webs which form the channels taper either upwardly or downwardly. A stream of steel from a ladle impacts the grid and is prevented from splashing or spraying in the tundish. The grid of this reference is designed to prevent splash upon the initial impact of molten steel with the grid in the tundish as distinguished from applicant's invention which suppresses turbulence and decreases slag entrainment in the molten steel after initial impact at relatively high levels (e.g., 1/2 to full height) of steel in the tundish. The reference does not disclose a critical ridge spacing of at least 2.0 times the ridge height.
U.S. Pat. No. 4,042,229, Eccleston discloses a tundish having lower beams 20 to confine the flow of molten metal to the impact area so as to minimize splash. Molten metal collects between the beams to form a reservoir until sufficient metal has been poured to overflow the beams. The beams are expendable in the sense that they are at least partially consumed and must be replaced each time the tundish is emptied of molten metal. The reference discloses beams for preventing splash as distinguished from applicant's invention which suppresses turbulence. The spacing and height of the beams are not disclosed in the reference.
U.S. Pat. No. 4,177,855, Duchateau, et al. discloses a flat pad for receiving the impact of a stream from a ladle to a tundish during the continuous casting of steel. The reference pad does not have ridges, whereas, the pad of applicant's invention does have ridges for suppressing turbulence.
U.S. Pat. No. 3,887,171, Neuhaus disclose having a trough 4 which together with the outer surface of a pouring tube 6 forms a path for higher flow velocity of steel upwardly into soft contact with the slag so as to drive impurities into the slag. The reference trough has walls which extend to the interface of slag and steel in the tundish as distinguished from the ridges of applicant's invention which are of substantially lower height. The reference also discloses barriers 7 which create a turbulence for further driving impurities into the slag. The reference barriers are displaced a substantial distance from the area of the pouring stream as distinguished from the ridges of applicant's invention. Also, the height and spacing of the barriers are not disclosed.
U.S. Pat. No. 3,865,175, Listhuber, et al. discloses a tundish (FIG. 4) having a casting tube 28 with a lateral opening close to its bottom. A shoulder 31 displaced from the lateral opening serves to deflect liquid steel vertically upward to increase its turbulence to within a controlled range so as to create a wave in the slag or casting powder layer. Non-metallic particles contained in the steel are flushed into the layer. The reference shoulder has a height of at least 4 cm. The reference does not disclose a ridge for suppressing turbulence as in applicant's invention, nor ridge spacing within a critical range for suppressing turbulence.
U.S. Pat. No. 4,711,429, Diederich, et al. discloses a tundish having walls spaced on opposite sides of the ladle pouring stream which extend upwardly to a height of at least 40 percent of the normal depth of metal in the tundish. The walls form a mixing box for creating turbulence in the metal in order to mix powdered alloy additions with the metal. The reference mixing box walls are designed to create turbulence rather than to suppress it. Also, the reference walls are higher than the ridges of applicant's invention.
Other references of interest are U.S. Pat. Nos. 4,993,692; 4,671,499; 4,372,542; and 4,043,543.